The manufacture of aminoguanidine bicarbonate (AGB) by reacting an aqueous solution of cyanamide with hydrazine followed by an addition of CO2 is known. Since the placing in contact of cyanamide with hydrazine in alkaline medium also leads to the dimerization of the cyanamide, it is necessary to use a large excess of cyanamide to achieve a suitable yield of aminoguanidine bicarbonate.
Thus, patent DD 689 191 teaches working with a 100% excess of cyanamide (i.e. cyanamide/hydrazine molar ratio=2/1) to obtain a yield (relative to the hydrazine used) of 80% of AGB, after reaction for 60 hours. An AGB yield of about 90% may be achieved after reaction for 27 hours when concentrated solutions of cyanamide and hydrazine are used with a 100% excess of cyanamide (DD 730 331).
Since cyanamide is a very expensive product, attempts to reduce this excess have been the subject of much research.
Specifically, patent SU 981 314 discloses a cyanamide/hydrazine molar ratio of between 1.25 and 1.8. It is mentioned that an AGB yield (relative to the hydrazine) of 95% is obtained with a cyanamide/hydrazine molar ratio of 1.8. It also teaches that the yields fall to 90% and 85% for molar ratios of 1.5 and 1.25, respectively.
The same trend has been observed by other authors. Thus, a fall of about 12 points in the yield was recorded when the cyanamide/hydrazine molar ratio went from 1.2 to 1 (DD 249 009).
The combined literature in this field encourages a person skilled in the art to work with an excess of cyanamide to obtain a yield of aminoguanidine bicarbonate.